The present invention relates to an improved fastening for the housing of a position measuring arrangement, and in particular, a low-vibration fastening arrangement. Position measuring arrangements are used to measure the relative position of two components such as a machine tool or a coordinate measuring machine.
German patent application number DE 25 05 587 C3 discloses a length measuring arrangement in which a measurement embodiment is fastened to a carrier body by an adhesive layer. The adhesive layer is elastic in order to avoid measurement inaccuracies caused by differing thermal expansion coefficients of the measurement embodiment and the carrier body. Because the measurement embodiment is held sufficiently fast by this exclusively elastic fastening layer, vibrations may be introduced in the measurement embodiment. Such vibrations disadvantageously affect the accuracy of measurement obtainable from a measuring arrangement so constructed.
In an attempt to avoid such a disadvantage, German Patent Application number DE 26 43 304 B2 proposed fastening the measurement embodiment to the carrier body so that the measurement embodiment rested on the border of a rectangular groove extending in the measuring direction. The measurement embodiment is joined with the carrier body in its middle section by an inelastic adhesive in the groove and at each end of the measurement embodiment by a highly elastic adhesive in the groove. This also has the disadvantage that the measurement embodiment, by reason of the elastic fastening to exclusively one surface of the carrier body, can be easily excited into vibration. In addition, the direct contact of the measurement embodiment with the border of the groove has the disadvantage that any production conditioned unevenness in the border or groove are transferred to the measurement embodiment which leads to measurement inaccuracies.
In an attempt to avoid vibration errors in the measuring arrangement, U.S. Pat. No. 4,444,504 discloses a measuring arrangement in which the measurement embodiment is not fastened to the carrier body only over one surface. Instead, a longitudinal groove is made in the carrier body for reception of the measurement embodiment. The measurement embodiment stands in direct contact with a side surface of the groove and with the base surface of the groove. The space between the other side surface of the groove and a surface of the measurement embodiment facing that side surface is filled with a rubber bar and an elastic adhesive. The measurement embodiment is further fastened by a clamp in a subregion of the measurement embodiment that is enclosed in the groove. A disadvantage with such a measuring arrangement is that the space filled by the elastic adhesive is relatively wide which again has a negative effect on the vibration properties of the measurement embodiment. Also, the use of the rubber bar to hold the measurement embodiment in place hampers free expansion between the measurement embodiment and the carrier body.
U.S. Pat. No. 4,492,033 discloses the use of individual rubber rods spaced apart from one another in the measuring direction X are provided to clamp the measurement in the housing. In addition, the measurement embodiment is supported on the bottom of the groove over a spacer. One problem with such a construction is that section-wise differing forces act on the measurement embodiment which can lead to inaccuracies in measurement.
German Patent Application number DE 36 05 789 C1 discloses a position measuring embodiment which is fastened in a groove formed in the carrier body. In particular, the measurement embodiment is embedded on its narrow side in a permanent elastic sealing compound. In order to ensure length expansion between the measurement embodiment and the carrier body, the sealing compound must be highly elastic. Such an elastic layer, however, introduces the negative influences of vibrations thereby compromising the stability of the measuring embodiment.
It is thus desirable to provide a position measuring arrangement in which unequal thermal properties of the measurement embodiment and the carrier body impart virtually no falsifications of the measuring result. In addition, it is desirable to provide a measuring arrangement in which the measurement embodiment is secured relatively vibration-free to the carrier body.